<template>
<ul class="list">
    <li class="item"
    v-for="item in letters"
    :key="item"
    :ref="item"
    @touchstart="handleTouchStart"
    @touchmove="handleTouchMove"
    @touchend="handleTouchEnd"
    @click="handleLetterClick"
    >
      {{item}}
    </li>
</ul>
</template>

<script>
  import BScroll from 'better-scroll'
  export default{
    name:'CityAlphabet',
    props:{
      cities:Object,
    },
    computed:{
      letters () {
        const letters = []
        for(let i in this.cities){
          letters.push(i)
        }
        return letters
      }
    },
    data () {
      return{
        touchStatus:false,
        startY:0,
        timer:null,
      }
    },
    updated(){
      this.startY = this.$refs['A'][0].offsetTop
    },
    methods:{
      handleLetterClick(e){
        this.$emit('change',e.target.innerText)
      },
      handleTouchStart () {
        this.touchStatus = true
      },
      handleTouchMove (e) {
        if(this.touchStatus){
          if(this.timer){ //函数截流
            clearTimeout(this.timer)
          }
          this.timer = setTimeout(() => { //函数截流 正在做手指滚动如果在16毫秒以内再次进行，清除掉上次的操作，立即执行现在的
            const touchY = e.touches[0].clientY - 79
              const index = Math.floor((touchY - this.startY) / 20)
            if(index >= 0 && index < this.letters.length){ //提升性能
              this.$emit('change',this.letters[index])
              }
          },16)
        }
      },
      handleTouchEnd () {
        this.touchStatus = false
      }
    }
  }
</script>

<style lang="stylus" scoped>
    @import '~styles/varibles.styl'
    .list
      display :flex
      flex-direction :column
      justify-content :center
      position :absolute
      top :1.58rem
      right :0
      bottom :0
      width :.4rem
      .item
        line-height :.4rem
        text-align :center
        color :$bgcolor
</style>
